My Tobii Pro X3-120 shows pupil size every third sample.

The pupil size is slightly different from dark and bright pupil, and more respondents will give better results in bright pupil mode.

To avoid non-significant variations in the pupil size between the dark and bright pupil illumination modes, the Pro X3-120 output gives the pupil size only for the bright pupil sample (i.e. every third sample).